RNA interference (RNAi) is an evolutionarily conserved, sequence specific mechanism triggered by double stranded RNA (dsRNA) that induces degradation of complementary target single stranded mRNA and “silencing” of the corresponding translated sequences (McManus and Sharp, Nature Rev. Genet. 3:737 (2002)). RNAi functions by enzymatic cleavage of longer dsRNA strands into biologically active “short-interfering RNA” (siRNA) sequences of about 21-23 nucleotides in length (Elbashir, et al., Genes Dev. 15:188 (2001)). siRNA can be used to downregulate or silence the transcription and translation of a gene product of interest, i.e., a target sequence.
Nucleic acids, like other macromolecules, can act as biological response modifiers, i.e., can induce immune responses in mammals upon in vivo administration. For example, poly(I:C)-LC has been identified as a potent inducer of interferon (IFN) as well as a macrophage activator and inducer of natural killer (NK) activity (Talmadge et al., Cancer Res. 45:1058 (1985); Wiltrout et al., J. Biol. Resp. Mod. 4:512 (1985); Krown, Sem. Oncol. 13:207 (1986); and Ewel et al., Canc. Res. 52:3005 (1992)). Unfortunately, toxic side effects have thus far prevented poly(I:C)-LC and other nucleic acids from becoming a useful therapeutic agent.
Several phosphorothioate modified oligodeoxynucleotides (ODN) have been reported to induce in vitro and in vivo B cell stimulation (Tanaka et al., J. Exp. Med. 175:597 (1992); Branda et al., Biochem. Pharmacol. 45:2037 (1993); McIntyre et al., Antisense Res. Develop. 3:309 (1993); and Pisetsky and Reich, Life Sciences 54:101 (1993)). However, none of these reports suggest a common structural motif or sequence element in these ODN that might explain their effects.
Recent reports have indicated that phosphorothioate-protected single-stranded RNA sequences comprising a GU-rich sequence derived from the U5 region of HIV-1 RNA complexed to the cationic lipid 1,2-dioleoyl-3-(trimethyammonium) (DOTAP) can induce expression of the cytokines IL-6, IL-12p40, TNF-α and IFN-α (see, e.g., Heil et al., Science 303:1526-1529 (2004)). In addition, U.S. Patent Publication No. 20030232074 and WO 03/086280 describe immunostimulatory RNA molecules, i.e., rRNA, tRNA, mRNA, and vRNA, comprising at least one guanine and at least one uracil. However, these reports do mention or suggest that siRNA has any immunostimulatory properties or that siRNA can be used to modulate an immune response.
Thus, there is a need for nucleic acid compositions that can be modified to modulate (i.e., increase or decrease) their immunostimulatory properties. The present invention addresses these and other needs.